Method and apparatus for measuring and reporting a rank and a precoding matrix  for multiple-input multiple-output communication

ABSTRACT

A method and apparatus for measuring and reporting a rank and/or a precoding matrix for multiple-input multiple-output (MIMO) communication are disclosed. A metric indicating a channel condition is measured and a rank is selected based on the metric. The metric may be a signal-to-interference and noise ratio (SINR), throughput, a block error rate (BLER), system capacity, a sum rate, or the like. An SINR for each radio block group (RBG) for each rank is calculated. A data rate is calculated for each RBG based on the SINR for each rank. An overall rate for all RBGs is calculated for each rank. At least one rank is selected based on the overall rate. At least one precoding matrix may be selected jointly with or separately from the at least one rank.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/077,620 filed Jul. 2, 2008 and 61/077,709 filed Jul. 2, 2008, whichare incorporated by reference as if fully set forth.

FIELD OF INVENTION

This application is related to wireless communications.

BACKGROUND

Spatial multiplexing of a wireless transmit/receive unit (WTRU) thatuses multiple-input multiple-output (MIMO) communication means mayinvolve determining a number of settings and parameters. The selectionof these settings and parameters desirably improve the quality andreliability of the MIMO communications. For example it is desirable forthe WTRU to determine a desired rank indicating a number of usefultransmission layers.

For open-loop spatial multiplexing mode, a reported rank equal to oneindicates a transmit diversity should be used for the MIMOcommunications, while a reported rank higher than one, (e.g., two,three, or four), indicates that large delay cyclic delay diversity (CDD)with two, three or four corresponding layers should be used.

For closed-loop spatial multiplexing mode, a reported rank indicates aclosed-loop precoding with the corresponding number of layers, (e.g.,one, two, three or four), should be used.

Rank measurement and generation may be performed for MIMO spatialmultiplexing for both open-loop and close-loop schemes.

Additionally, the determination of a precoding matrix index (PMI) may bedesirable for MIMO communications by a WTRU.

The present application includes several example methods and apparatusesfor selecting and reporting ranks and PMIs for MIMO communication by aWTRU.

SUMMARY

A method and apparatus for measuring and reporting a rank and/or aprecoding matrix for multiple-input multiple-output (MIMO) communicationare disclosed. A metric indicating a channel condition is measured and arank is selected based on the metric. The metric may be asignal-to-interference and noise ratio (SINR), throughput, a block errorrate (BLER), system capacity, a sum rate, or the like. An SINR for eachradio block group (RBG) for each rank is calculated. A data rate iscalculated for each RBG based on the SINR for each rank. An overall ratefor all RBGs is calculated for each rank. At least one rank is selectedbased on the overall rate. At least one precoding matrix may be selectedjointly with or separately from the at least one rank.

BRIEF DESCRIPTION OF THE DRAWINGS

A more detailed understanding may be had from the following description,given by way of example in conjunction with the accompanying drawingswherein:

FIG. 1 is a schematic block diagram illustrating an example long termevolution advanced (LTE-A) telecommunications system according to thepresent application;

FIG. 2 is a schematic block diagram illustrating an example wirelesstransmit/receive unit (WTRU) and Node-B according to the presentapplication;

FIG. 3 is a flowchart illustrating an example method for selecting arank for multiple-input multiple-output (MIMO) communication by awireless transmit/receive unit (WTRU) according to the presentapplication;

FIG. 4 is a schematic block diagram illustrating an example WTRUconfigured to select a rank and/or precoding matrix index (PMI) for MIMOcommunication according to the present application;

FIG. 5 is a flowchart illustrating an example method for selecting ajoint PMI and rank for MIMO communication by a WTRU according to thepresent application; and

FIG. 6 is a flowchart illustrating an example method for selecting a PMIfor MIMO communication by a WTRU according to the present application.

DETAILED DESCRIPTION

When referred to hereafter, the terminology “wireless transmit/receiveunit (WTRU)” includes but is not limited to a user equipment (UE), amobile station, a fixed or mobile subscriber unit, a pager, a cellulartelephone, a personal digital assistant (PDA), a computer, or any othertype of user device capable of operating in a wireless environment. Whenreferred to hereafter, the terminology “base station” includes but isnot limited to a Node-B, a site controller, an access point (AP), or anyother type of interfacing device capable of operating in a wirelessenvironment.

FIG. 1 shows a wireless communication system 100 including a pluralityof WTRUs 110, a Node-B 120, a controlling radio network controller(CRNC) 130, a serving radio network controller (SRNC) 140, and a corenetwork 150. The Node-B 120 and the CRNC 130 may collectively bereferred to as the UTRAN.

As shown in FIG. 1, the WTRUs 110 are configured for multiple-inputmultiple-output (MIMO) communication with the Node-B 120, which is incommunication with the CRNC 130 and the SRNC 140. Although three WTRUs110, one Node-B 120, one CRNC 130, and one SRNC 140 are shown in FIG. 1,it should be noted that any combination of wireless and wired devicesmay be included in the wireless communication system 100.

FIG. 2 is a functional block diagram 200 of a WTRU 110 and the Node-B120 of the wireless communication system 100 of FIG. 1. As shown in FIG.2, the WTRU 110 is in communication with the Node-B 120 and both may beconfigured to perform methods of MIMO rank selection and/or PMIselection in a WTRU.

In addition to the components that may be found in a typical WTRU, theWTRU 110 includes a processor 115, a receiver 116, a transmitter 117,and an antenna 118. The processor 115 is configured to perform a methodof rank and/or precoding matrix index (PMI) measurement and selection ina WTRU. The receiver 116 and the transmitter 117 are in communicationwith the processor 115. The antenna 118 is in communication with boththe receiver 116 and the transmitter 117 to facilitate the transmissionand reception of wireless data.

In addition to the components that may be found in a typical basestation, the Node-B 120 includes a processor 125, a receiver 126, atransmitter 127, and an antenna 128. The processor 125 may be configuredto perform methods of selecting MIMO rank selection and/or PMI selectionin a WTRU. The receiver 126 and the transmitter 127 are in communicationwith the processor 125. The antenna 128 is in communication with boththe receiver 126 and the transmitter 127 to facilitate the transmissionand reception of wireless data.

Example methods and apparatuses for enhanced MIMO rank and/or PMImeasurement and generation are disclosed. Example methods andapparatuses for feedback and reporting of the selected PMI and rankinformation indicative of the selected rank are also disclosed. Severalexample methods for enhanced MIMO rank and/or PMI measurement andgeneration are described in detail; however, these examples are notintended to be limiting.

During MIMO operation the rank information are desirably measured andgenerated at a receiver or a WTRU and the generated rank information arefed back to a transmitter or an eNodeB. Rank measurement and generationmay be performed for MIMO spatial multiplexing for both open-loop andclosed-loop schemes.

For spatial multiplexing, the WTRU determines a rank indicating a numberof useful transmission layers. For open-loop spatial multiplexing mode,a reported rank equal to one indicates that a transmit diversity shouldbe used, while a reported rank higher than one, (e.g., two, three, orfour), indicates that large delay cyclic delay diversity (CDD) with two,three or four corresponding layers should be used.

For closed-loop spatial multiplexing mode, the reported rank indicates aclosed-loop precoding with the corresponding number of layers, (e.g.,one, two, three or four), should be used.

Table 1 summaries the rank value and the corresponding spatialmultiplexing schemes for both open-loop and closed-loop modes.

TABLE 1 Rank Corresponding Corresponding (Number of Open-Loop SpatialClosed-Loop Spatial layers) Multiplexing Scheme Multiplexing Scheme 1Transmit Diversity Precoding (one layer) 2 Large delay CDD (twoPrecoding (two layers) layers) 3 Large delay CDD (three Precoding (threelayers) layers) 4 Large delay CDD (four Precoding (four layers) layers)

The example methods and apparatuses described below may be to determinethe desired rank for the current channel condition according topredetermined criteria. These criteria may include asignal-to-interference and noise ratio (SINR)-based metric. Othercriteria, such as throughput, block error rate (BLER), system capacity,sum rate-based metrics, etc. may also be used.

Often a single rank measurement and generation, as well as a single rankfeedback and reporting are used for a large bandwidth, (e.g., the entirebandwidth). Alternatively, the same methods may be extended to measureand generate multiple ranks for portions of the bandwidth, if desired.For example, a rank for each sub-band or group of sub-bands may bemeasured and generated. Accordingly, these multiple ranks, (i.e., eachfor a sub-band or a group of sub-bands), may be fed back and reported aspart of a multi-band rank or multi-rank measurement and reportingscheme.

For a 2×2 antenna configuration, 1 bit may be required to represent therank, (either rank=1 or 2). For a 4×4 antenna configuration, 2 bits maybe required to represent the rank, (i.e., rank=1, 2, 3 and 4).

TABLE 2 Number of Bits for Rank Feedback 2 × 2 MIMO 1 bit  4 × 4 MIMO 2bits

FIG. 3 is a flowchart illustrating an example method for rankmeasurement and generation in a WTRU using MIMO communication.

For each rank of a plurality of possible ranks, a value of a metricindicative of the channel condition of the MIMO communication by theWTRU using that rank is determined, step 300. This metric may be basedon one or more measured quantities indicative of the channel condition,such as signal-to-interference-and-noise ratio (SINR), throughput, ablock error rate (BLER), system capacity, or a sum rate.

The rank may then be selected from the plurality of ranks based on thevalues of the metric, step 302.

For example, if the metric is a sum rate based on SINR, the SINR may becomputed based on the channel measurement, (e.g., channel estimation)for each rank, or each sub-band (i.e., resource block (RB)), or resourceblock group (RGB) thereof. A rate for each sub-band, or RBG, may then becomputed based on the measured SINR for the sub-band, or RBG. Theoverall sum rate is then computed for all sub-bands for each rank byadding the sub-band rates, or RBG rates, of the individual sub-bands, orRBGs. The desired rank is selected based on this overall sum rate, i.e.the rank having the largest sum rate is selected.

The value of the metric for each rank may also be determined bydetermining a stream sub-value of the metric for each spatial streamused for the MIMO communication. These stream sub-values of the metricmay then be added to determine the value of the metric.

In open-loop spatial multiplexing mode rank one corresponds to transmitdiversity scheme and rank two or higher corresponds to large delay CDDscheme. A sum rate-based metric is used herein as an example. Theseexample methods involve computing a sum rate for each rank. Whencomputing the sum rate for rank one, a transmit diversity scheme isassumed. The SINR is measured and computed assuming transmit diversityscheme is used. When computing overall rates for rank two or higher,large delay CDD is assumed. The SINR is measured and computed assuminglarge delay CDD with proper number of layers. Overall rate is thencomputed based on the computed SINR for each rank.

The SINR for the j-th spatial stream and rank p, i.e. the rank PJ RBGSINR, in the g-th RBG may be expressed as follows:

$\begin{matrix}{{\gamma_{g}^{({p,j})} = \frac{{w_{g,j,j}^{(p)}}^{2}}{{\sum\limits_{{k = 1},{k \neq j}}^{Ns}{w_{g,j,k}^{(p)}}^{2}} + {\sigma_{n}^{2}{\sum\limits_{k = 1}^{Ns}{z_{g,j,k}^{(p)}}^{2}}}}},{p = 2},3,\ldots \mspace{11mu},{P;}} & {{Equation}\mspace{14mu} (1)}\end{matrix}$

where w_(g,j,k) ^((p)) and z_(g,j,k) ^((p)) is the (j,k)-th element ofmatrix W_(g) ^((P)) and Z_(g) ^((p)), respectively and Ns is the numberof data streams. P=2 for 2×2 MIMO and P=4 for 4×4 MIMO.

Matrix Z_(g) ^((p)) may be obtained by:

Z _(g) ^((p)) =H _(g) ^(p) ^(H) (H _(g) ^(p) H _(g) ^(P) ^(H) +R_(In))⁻¹;  Equation (2)

where R_(In) is the covariance matrix of interference and/or noise.

The matrix W_(g) ^((p)) may be obtained by:

W_(g) ^((p))=Z_(g) ^((p))H_(g) ^((p)).  Equation (3)

The channel H_(g) ^((p)) is the averaged effective channel matrix ofrank p for the g-th RBG. Large delay CDD may use several matrices. Largedelay CDD may use two matrices for two layers, three matrices for threelayers and four matrices for four layers. In addition, the matrix maycycle through or hop through different version of the matrix forobtaining diversity gain. The SINR may be averaged over all matrices.The effective channel matrix may be averaged over all matrices toproduce a single averaged matrix. The large delay matrix is expressed asfollows:

T=W(i)D(i)U;  Equation (4)

where W(i) is a matrix pre-selected from a precoding codebook. W(i) maybe a single matrix or selected from a subset of precoding codebook. D(i)is a matrix that may depend on the data symbol index i. U is apredetermined fixed matrix. If there is a single matrix for W(i), for 2transmit antenna configuration there may be two matrices for D(i) if itis two layers as follows:

$\begin{matrix}{{{D(i)} = \begin{bmatrix}1 & 0 \\0 & ^{{- j}\; 2\; \pi \; {/2}}\end{bmatrix}},{i = 0},1.} & {{Equation}\mspace{14mu} (5)}\end{matrix}$

If there are multiple matrices for W(i), for 4 transmit antennaconfiguration assume W(i)ε{C₁, C₂, . . . , C_(L)}, each W(i) combineswith a D(i) to produce a new matrix T. There may be three matrices forD(i) if it is three layers as follows:

$\begin{matrix}{{{D(i)} = \begin{bmatrix}1 & 0 & 0 \\0 & ^{{- j}\; 2\; \pi \; {/3}} & 0 \\0 & 0 & ^{{- j}\; 4\; \pi \; {/3}}\end{bmatrix}},{i = 0},1,2.} & {{Equation}\mspace{14mu} (6)}\end{matrix}$

There may be four matrices for D(i) if it is four layers as follows:

$\begin{matrix}{{{D(i)} = \begin{bmatrix}1 & 0 & 0 & 0 \\0 & ^{{- j}\; 2\; \pi \; {/4}} & 0 & 0 \\0 & 0 & ^{{- j}\; 4\; \pi \; {/4}} & 0 \\0 & 0 & 0 & ^{{- j}\; 6\; \pi \; {/4}}\end{bmatrix}},{i = 0},1,2,3.} & {{Equation}\mspace{14mu} (7)}\end{matrix}$

The SINR should be averaged over all matrices D(i) and/or W(i) dependingon the way that matrices W(i) are assigned. Once matrix T is determined,the effective channel may be computed accordingly. For each matrix Tobtained, a corresponding SINR is computed. Denote the computed SINR asSINR₁, SINR₂, . . . , SINR_(N). The averaged SINR may be obtained asfollows:

$\begin{matrix}{{{{SIN}\; R_{avg}} = {\sum\limits_{i = 1}^{N}{\alpha_{i}{SIN}\; {R_{i}.}}}}\;} & {{Equation}\mspace{14mu} (8)}\end{matrix}$

where α_(i) is the weight coefficient for averaging processing.

The SINR for transmit diversity using Alamouti scheme for linear minimummean square error (LMMSE) or maximum ratio combining (MRC) may becomputed for two transmit and one receiving antennas as follows:

$\begin{matrix}{\gamma_{g}^{({1,j})} = {\frac{{h_{g\; 11}}^{2} + {h_{g,12}}^{2}}{\sigma_{n}^{2}}.}} & {{Equation}\mspace{14mu} (9)}\end{matrix}$

Similarly the SINR for transmit diversity using other schemes may beproperly computed. Once the SINR is computed, the overall rate may becomputed accordingly:

$\begin{matrix}{\gamma_{g}^{({1,1})} = \frac{\sum\limits_{m = 1}^{N_{r}}{\sum\limits_{n = 1}^{N_{t}}{h_{g,m,n}}^{2}}}{\sigma_{n}^{2}}} & {{Equation}\mspace{14mu} \left( {9\; a} \right)}\end{matrix}$

where γ_(g) ^((1,1)) is the SINR for rank 1 and one stream for transmitdiversity. Nt and Nr are the number of antennas at transmitter andreceiver respectively.

The sum rate of all spatial streams and all sub-bands or RBGs for rank pmay be expressed as follows:

$\begin{matrix}{{\prod\limits^{(p)}{= {\sum\limits_{g}^{N_{g}}{\sum\limits_{j = 1}^{p}{\log \; 2\left( {1 + \gamma_{g}^{({p,j})}} \right)}}}}},{p = 1},2,\ldots \mspace{11mu},{P;}} & {{Equation}\mspace{14mu} (10)}\end{matrix}$

where Ng is the number of RBGs in the configured system bandwidth. Asingle rank is selected. The rank that maximizes the overall ratesacross all RBGs is selected,

$\begin{matrix}{{{Rank}_{sel} = {\arg \; {\max\limits_{p \in \Omega_{p}}\prod\limits^{(p)}}}};} & {{Equation}\mspace{14mu} (11)}\end{matrix}$

where Ω_(p) is a set of allowable rank, i.e., Ω_(p)={1, 2, . . . , P}.Typically, for 4×4 MIMO configuration Ω_(p)={1,2,3,4}. Maximum rankequals four. If p=1 is selected, it indicates transmit diversity ispreferred. If p=2, 3 or 4 are selected, it indicates large delay CDDwith two, three or four layers are preferred.

In closed-loop spatial multiplexing mode, rank x corresponds to a smalldelay CDD scheme with x layers. Value of x may be one to four for 4×4antenna configuration and one or two for 2×2 antenna configuration. Asum rate-based metric is used herein as an example. These examplemethods involve computing the overall rate for each rank. The SINR ismeasured and computed assuming small delay CDD with proper number oflayers.

When sum rate criteria are used, the SINR may be desirably computed foreach given rank. The sum rate is then computed based on the computedSINR. An example procedure for measuring and generating rank informationusing sum rate criteria is described as follows:

Step 1: The bandwidth is partitioned into several sub-bands or RBGs ofproper size for measurement and channel averaging purpose. The size maybe chosen by implementation for optimum performance.

Step 2: For each sub-band or RBG, an SINR for each rank is computed. Therank is 1, 2, . . . , P, where P is the maximum rank for a given antennaconfiguration of an eNodeB and a WTRU. For example if antennaconfiguration is 4×4, the maximum rank P=4. The SINR is computed foreach spatial stream for each rank.

Step 3: For each sub-band or RBG, a data rate is computed for each rankbased on the SINR computed in each sub-band or RBG. If the rank is one,data rate is computed for one layer.

Step 4: A corresponding overall sum rate of all sub-bands or RBGs iscomputed for each rank=1, 2, . . . , P using the previously computed sumrates for each sub-band or RBG.

Step 5: A rank that has the highest overall sum rate is selected.

Denote H_(g) ^((p)) the averaged effective channel matrix of rank p forthe g-th sub-band or RBG. The averaged effective channel matrix of rankp for the g-th sub-band or RBG, H_(g) ^((p)), is obtained by amultiplication of channel matrix H_(g) and precoding matrix T_(g) ^((p))of rank p for the g-th sub-band or RBG, as follows:

H_(g) ^((p))=H_(g)T_(g) ^((p)).  Equation (12)

The SINR for the j-th spatial stream for rank p in the g-th sub-band orRBG may be expressed as follows:

$\begin{matrix}{\gamma_{g}^{({p,j})} = {\frac{1}{{\sigma_{n}^{2}\left\lbrack \left( {{H_{g}^{p}{H_{g}^{p}}^{H}} + {\sigma_{n}^{2}I}} \right\rbrack^{- 1} \right\rbrack}_{j,j}} - 1.}} & {{Equation}\mspace{14mu} (13)}\end{matrix}$

Alternative measurement for SINR for the j-th spatial stream and rank pin the g-th sub-band or RBG may be expressed as follows:

$\begin{matrix}{{\gamma_{g}^{({p,j})} = \frac{{w_{g,j,j}^{(p)}}^{2}}{{\sum\limits_{{k = 1},{k \neq j}}^{Ns}{w_{g,j,k}^{(p)}}^{2}} + {\sigma_{n}^{2}{\sum\limits_{k = 1}^{Ns}{z_{g,j,k}^{(p)}}^{2}}}}};} & {{Equation}\mspace{14mu} (14)}\end{matrix}$

where w_(g,j,k) ^((p)) and z_(g,j,k) ^((p)) is the (j,k)-th element ofmatrix W_(g) ^((p)) and Z_(g) ^((p)) respectively. Matrix Z_(g) ^((p))may be obtained by:

Z _(g) ^((p)) =H _(g) ^(p) ^(H) (H _(g) ^(p) H _(g) ^(p) ^(H) +R_(In))⁻¹;  Equation (15)

where R_(In) is the covariance matrix of interference and noise. Thematrix W_(g) ^((p)) may be obtained by:

W_(g) ^((p))=Z_(g) ^((p))H_(g) ^((p)).  Equation (16)

The sum rate of all spatial streams and all sub-bands or RBGs for rank pmay be expressed as follows:

$\begin{matrix}{{\prod\limits^{(p)}{= {\sum\limits_{g}^{N_{g}}{\sum\limits_{j = 1}^{p}{\log \; 2\left( {1 + \gamma_{g}^{({p,j})}} \right)}}}}},{p = 1},2,\ldots \mspace{11mu},{P;}} & {{Equation}\mspace{14mu} (17)}\end{matrix}$

where Ng is the number of sub-bands or RBGs in the configured systembandwidth. A single value for rank is selected for a large bandwidth.The rank that maximizes the overall rates across all sub-bands or RBGsis selected, as follows:

$\begin{matrix}{{{Rank}_{sel} = {\arg \; {\max\limits_{\underset{p \in \Omega_{p}}{p,}}\prod\limits^{(p)}}}};} & {{Equation}\mspace{14mu} (18)}\end{matrix}$

where Ω_(p) is a set of allowable ranks, (i.e., Ω_(p)={1, 2, . . . ,P}). For 4×4 MIMO configuration, Ω_(p)={1,2,3,4} and the maximum rank isfour.

It is contemplated that a second rank which has the second highestoverall rates or the second best metric may also be selected. In thisexample embodiment, the WTRU may signal the first and second ranks thathave the highest overall rates or the best metric (primary rank) and thesecond highest overall rates or the second best metric (secondary rank),respectively. This allows the network to have more flexibility to assignthe rank and schedule the resources for data transmission if the firstrank, or primary rank, may not be desirable from a network perspective.The network may signal to the WTRU, indicating whether the second bestrank should be reported or not. Such indication may be donesemi-statically or dynamically.

In a further example embodiment, the best K ranks may be selected whichhave the K highest overall rates or the K best metric. The WTRU maysignal the best K ranks to allow network to have flexibility to assignthe number of data streams and schedule the resources for datatransmission if the some rank(s) is(are) not desirable from a networkperspective. The network may signal to the WTRU, indicating whether thebest K ranks should be reported or not. Such indication may be donesemi-statically or dynamically.

Once the desired rank for MIMO communication by the WTRU has beenselected rank information indicative of the selected rank is transmittedby the WTRU, step 304. When reporting rank information, the WTRUdesirably reports a single instance of the number of useful transmissionlayers.

For each rank reporting interval for closed-loop spatial multiplexingmode, a WTRU determines a rank from the supported set of rank values forthe corresponding antenna configuration of the eNodeB and the WTRU andreports the number in each rank report.

For each rank reporting interval for open-loop spatial multiplexingmode, a WTRU determines a rank from the supported set of rank values forthe corresponding antenna configuration of the eNodeB and the WTRU andreports the number in each rank report.

A WTRU may perform aperiodic rank reporting using the physical uplinkshared channel (PUSCH) upon receiving an indication sent in a downlinkscheduling grant. The minimum reporting interval for aperiodic reportingof rank information may be one subframe. A WTRU may be semi-staticallyconfigured by higher layers to feed back rank information on the PUSCHusing one of the reporting modes.

The rank value that the WTRU determines may also be used for selectingthe precoding matrix index (PMI) and the calculation of the channelquality index (CQI). Rank and PMI may be computed and determinedseparately. Alternatively, the rank and PMI may be computed anddetermined jointly by WTRU.

A WTRU may be semi-statically configured by higher layers toperiodically feed back rank information on the physical uplink controlchannel (PUCCH) using one of the reporting modes. The reporting intervalof rank information reporting may desirably be an integer multiple ofwideband CQI and/or PMI reporting period. The offset between rankinformation reporting and wideband CQI and/or PMI reporting instances isconfigured and the same or different offset values may be used for rankand CQI/PMI reporting. Both the reporting interval and offset may beconfigured by higher layers. In case of collision of rank informationand wideband CQI and/or PMI reporting, it may be desirable for the rankinformation to be transmitted and the wideband CQI and/or PMI reportingto be discarded. In this case, rank information is considered to be moreimportant than CQI and/or PMI.

In addition to selecting a desired rank for MIMO communication be aWTRU, it may be desirable to select a PMI from among a plurality ofpossible PMIs as well. For MIMO communication with precoding, twoexample approaches for PMI and rank selection are described in detailbelow—a joint approach for rank and PMI selection and generation, and aseparate measurement approach for rank and PMI selection and generation.In the joint approach, rank and PMI are measured and selected jointly.In the separate measurement approach, rank and PMI are generatedseparately and individually. Thus, the joint approach uses one stageprocessing while separate measurement approach uses two stagesprocessing.

In the separate measurement approach, for each PMI of a plurality ofPMIs, a PMI value of the metric using the selected rank may bedetermined. One PMI is then selected from the plurality of PMIs based onthe PMI values of the metric and the selected PMI is then transmitted.

As an example of this approach, continuing the example described aboveusing SINR and a sum rate as the metric, an SINR for LMMSE may becomputed for each sub-band or RBG for the selected rank assumingopen-loop MIMO communication. A corresponding data rate for eachsub-band or RBG for each rank may then calculated using the computedSINR for that sub-band or RBG. The overall rates for all sub-bands orRBGs are summed up. The rank that produces the highest overall sum rateis then selected. Once the rank is selected, a corresponding precodingmatrix is selected as follows. Given the selected rank, an SINR iscomputed for each PMI for each RBG. An overall sum rate is calculated bysumming up the data rates of all RBGs for each precoding matrix. Thedesired precoding matrix is then selected for the selected rank in openloop MIMO as the precoding matrix having the highest sum rate for theselected rank.

An SINR for LMMSE may be computed for each RBG for given rank andprecoding matrix. The corresponding data rate for each RBG may becalculated using the computed SINR for that RBG. The overall rates forall RBGs may be calculated for the given rank and precoding matrix. TheSINR for the j-th spatial stream for given precoding matrix F_(i) andrank p may be expressed as follows:

$\begin{matrix}{{\gamma_{g}^{({p,j})} = \frac{{w_{g,j,j}^{(p)}}^{2}}{{\sum\limits_{{k = 1},{k \neq j}}^{Ns}{w_{g,j,k}^{(p)}}^{2}} + {\sigma_{n}^{2}{\sum\limits_{k = 1}^{Ns}{z_{g,j,k}^{(p)}}^{2}}}}};} & {{Equation}\mspace{14mu} (19)}\end{matrix}$

where w_(g,j,k) ^((p)) and z_(g,j,k) ^((p)) is the (j,k)-th element ofmatrix W_(g) ^((p)) and Z_(g) ^((p)) respectively. The channel H_(g)^((p)) is the averaged channel matrix of rank p for the g-th RBG.

Matrix Z_(g) ^((p)) may be obtained by:

Z _(g) ^((p)) =H _(g) ^(p) ^(H) (H _(g) ^(p) H _(g) ^(p) ^(H) +R_(In))⁻¹;  Equation (20)

where R_(In) is the covariance matrix of interference and/or noise.

The matrix W_(g) ^((p)) may be obtained by:

W_(g) ^((p))=Z_(g) ^((p))H_(g) ^((p)).  Equation (21)

For single PMI measurement, the sum rate of all spatial streams and allRBGs for rank p and PMI i may be expressed as follows:

$\begin{matrix}{{\overset{(p)}{\prod\limits_{i}}{= {\sum\limits_{g}^{N_{g}}{\sum\limits_{j = 1}^{Ns}{\log \; 2\left( {1 + \gamma_{g,i}^{({p,j})}} \right)}}}}};} & {{Equation}\mspace{14mu} (22)}\end{matrix}$

where Ns is the maximum number of data streams and Ng is the number ofRBGs to be considered.

The PMI that maximizes the overall rates across all RBGs is selected,

$\begin{matrix}{{PMI}_{sel} = {\arg \; {\max\limits_{i}{\underset{i}{\overset{\;}{\prod\limits^{(p)}}}.}}}} & {{Equation}\mspace{14mu} (23)}\end{matrix}$

It is contemplated that a second PMI which has the second highestoverall rates or the second best metric may also be selected. In thisexample embodiment, the WTRU may signal the first and second PMIs thathave the highest overall rates or the best metric (primary rank) and thesecond highest overall rates or the second best metric (secondary rank),respectively. This allows the network to have more flexibility to assignthe PMI and schedule the resources for data transmission if the primaryPMI is not desirable from a network perspective. The network may signalto the WTRU, indicating whether the second best PMI should be reportedor not. Such indication may be done semi-statically or dynamically.

In a further example embodiment, the best K PMIs may be selected whichhave the K highest overall rates or the K best metrics. The WTRU maysignal the best K PMIs to allow network to have flexibility to assignthe PMI and schedule the resources for data transmission if the somePMIs are not desirable from a network perspective. The network maysignal to the WTRU, indicating whether the best K PMIs should bereported or not. Such indication may be done semi-statically ordynamically.

For multiple PMI measurement, the sum rate of all spatial streams forg-th RB and PMI i given rank p may be expressed as follows:

$\begin{matrix}{\underset{g,i}{\overset{\;}{\prod\limits^{(p)}}}{= {\sum\limits_{j = 1}^{Ns}{\log \; 2{\left( {1 + \gamma_{g,i}^{({p,j})}} \right).}}}}} & {{Equation}\mspace{14mu} (24)}\end{matrix}$

The PMI that maximizes the rate for each RBG is selected,

$\begin{matrix}{{PMI}_{g,{sel}} = {\arg \; {\max\limits_{i}{\underset{g,i}{\overset{\;}{\prod\limits^{(p)}}}.}}}} & {{Equation}\mspace{14mu} (25)}\end{matrix}$

It is contemplated that a second PMI which has the second highestoverall rates or the second best metric may be selected for each subbandor RBG. In this example embodiment, the WTRU may signal the first andsecond PMIs that have the highest overall rates or the best metric(primary rank) and the second highest overall rates or the second bestmetric (secondary rank), respectively. This allows the network to havemore flexibility to assign the PMI and schedule the resources for datatransmission for each RBG if the primary PMI is not desirable from anetwork perspective for that particular RBG. The network may signal tothe WTRU, indicating whether the second best PMI for certain RBG or RBGsshould be reported or not. Such indication may be done semi-staticallyor dynamically.

In a further example embodiment, the best K PMIs for each subband or RBGmay be selected which have the K highest overall rates or the K bestmetrics for that subband or RBG. The WTRU may signal the best K PMIs foreach subband or RBG to allow network to have flexibility to assign thePMI and schedule the resources for data transmission if some of PMIs arenot desirable from a network perspective. The network may signal to theWTRU, indicating whether the best K PMIs for each subband or RBG shouldbe reported or not. Such indication may be done semi-statically ordynamically.

For MIMO communication with precoding, the rank measurement may beperformed jointly with precoding matrix selection. FIG. 5 illustrates anexample method for joint selection of a PMI from a plurality of PMIs anda rank from a plurality of ranks for MIMO communication by a WTRU. Inthis example method only one stage joint processing is used.

For each PMI and rank combination, a value of a metric indicative of achannel condition of the MIMO communication by the WTRU using that PMIand rank is determined, step 500. This metric may be based on one ormore measured quantities indicative of the channel condition, such assignal-to-interference-and-noise ratio (SINR), throughput, a block errorrate (BLER), system capacity, or a sum rate.

The PMI and rank combination may then be selected from the plurality ofPMI and rank combinations based on the values of the metric, step 502.

For example, if the metric is a sum rate based on SINR, the SINR may becomputed based on the channel measurement, (e.g., channel estimation)for each PMI and rank combination. Alternatively, the SINR for each PMIand rank combination on each sub-band (i.e., resource block (RB)), orresource block group (RGB) may be measured. A rate for each sub-band, orRBG, may then be computed based on the measured SINR for the sub-band,or RBG. The overall sum rate is then computed for all sub-bands for eachPMI and rank combination by adding the sub-band rates, or RBG rates, ofthe individual sub-bands, or RBGs. The desired PMI and rank combinationis selected based on this overall sum rate, i.e. the PMI and rankcombination having the largest sum rate is selected.

Using Equations 19-21, the sum rate of all spatial streams and all RBGsfor rank p and PMI i may be expressed as follows:

$\begin{matrix}{{\underset{i}{\overset{\;}{\prod\limits^{(p)}}}{= {\sum\limits_{g}^{N_{g}}\; {\sum\limits_{j = 1}^{Ns}{\log \; 2\left( {1 + \gamma_{g,i}^{({p,j})}} \right)}}}}};} & {{Equation}\mspace{14mu} (26)}\end{matrix}$

where Ns is the maximum number of data streams. A single rank isselected. The rank and PMI that maximize the overall rates across allRBGs are jointly selected,

$\begin{matrix}{\left\lbrack {{Rank}_{sel},{PMI}_{sel}} \right\rbrack = {\arg \; {\max\limits_{p,i}{\underset{i}{\overset{\;}{\prod\limits^{(p)}}}.}}}} & {{Equation}\mspace{14mu} (27)}\end{matrix}$

Once the desired PMI and rank combination for MIMO communication by theWTRU has been selected, the selected PMI and rank information indicativeof the selected rank is transmitted by the WTRU, step 504.

It is contemplated that a second combination of PMI and rank which hasthe second highest overall rates or the second best metric may also beselected. In this example embodiment, the WTRU may signal both the firstand second combination of PMI and rank that have the highest overallrates or the best metric (primary rank/PMI) and the second highestoverall rates or the second best metric (secondary rank/PMI),respectively. This allows the network to have more flexibility to assignthe rank and PMI and schedule the resources for data transmission if theprimary combination of rank and PMI is not desirable from a networkperspective. For multi-RBG combinations of rank and PMI measurement, abest combination of rank and PMI is selected for each RBG. In addition,a second best combination of rank and PMI may be selected for each RBG.The network may signal to the WTRU, indicating whether the second bestcombination of rank and PMI should be reported or not. Such indicationmay be done semi-statically or dynamically.

In a further example embodiment, the best K combinations of rank and PMImay be selected which have the K highest overall rates or the K bestmetrics. The WTRU may signal the best K combinations of rank and PMI toallow network to have flexibility to assign the number of data streamsand PMI and schedule the resources for data transmission if the somerank and/or PMI are not desirable from a network perspective. Thenetwork may signal to the WTRU, indicating whether the best Kcombinations of rank and PMIs should be reported or not. Such indicationmay be done semi-statically or dynamically.

A WTRU may perform aperiodic PMI and rank reporting using the PUSCH uponreceiving an indication sent in a downlink scheduling grant. The minimumreporting interval for aperiodic reporting of PMI and rank informationmay be one subframe. A WTRU may be semi-statically configured by higherlayers to feed back PMI and rank information on the PUSCH using one ofthe reporting modes. The PMI and rank value that the WTRU determines mayalso be used for the calculation of the CQI.

A WTRU may be semi-statically configured by higher layers toperiodically feed back PMI and rank information on the PUCCH using oneof the reporting modes. The reporting interval of rank informationreporting may desirably be an integer multiple of wideband CQI and/orPMI reporting period. The offset between rank information reporting andwideband CQI and/or PMI reporting instances is configured and the sameor different offset values may be used for rank and CQI/PMI reporting.Both the reporting interval and offset may be configured by higherlayers. In case of collision of rank information and wideband CQI and/orPMI reporting, it may be desirable for the rank information to betransmitted and the wideband CQI and/or PMI reporting to be discarded.

FIG. 4 illustrates an example WTRU configured for MIMO communicationthat may be used to perform the various method of the presentapplication. The example WTRU includes: receiver 400; channel conditionprocessor 404, which is coupled to receiver 400; selection processor406, which is coupled to channel condition processor 404; transmitter408, which is coupled to selection processor 406; and antenna array 402,which is coupled to receiver 400 and transmitter 408.

Receiver 400 is configured to receive a signal detected by antenna array402, which it then relays to channel condition processor 404. Channelcondition processor 404 is configured to determine the value of a metricindicative of channel condition based on the received signal. Channelcondition processor 404 may be configured such that the metric valuesare determined for each of a plurality of ranks, a plurality of PMIs, ora plurality of PMI and rank combinations. The metric may be based on atleast one of SINR, throughput, BLER, system capacity, or a sum rate.

Further, for each rank of the plurality of PMIs and/or ranks, channelcondition processor 404 may be configured to determine the value of themetric determining a stream sub-value (or RBG sub-value) of the metricfor each spatial stream (or RBG) used for the MIMO communication andthen adding the stream sub-values (or RBG sub-values) of the metric todetermine the value of the metric.

Selection processor 406 (or joint selection processor in the case ofjoint selection of PMI and rank) is configured to select the PMI and/orrank from the plurality of ranks based on the values of the metricdetermined by channel condition processor 404.

Transmitter 408 is configured to transmit the selected PMI and/or rankinformation indicative of the selected rank.

It is noted that channel condition processor 404, selection processor406, and transmitter 408 may be configured to perform any of the channelcondition determination, selection, and transmission proceduresdisclosed above with respect to the example methods of FIGS. 3 and 5 orbelow with respect to the example methods of FIG. 6.

FIG. 6 illustrates an example method for selecting a PMI from aplurality of PMIs for MIMO communication by a WTRU. This example methodmay include procedures for PMI selection and generation for advancedWTRUs, such as advanced WTRUs using minimum mean square error (MMSE)successive interference cancellation (SIC). Example procedures for PMIselection and generation for different MIMO precoding configurations,such as wideband precoding, M-sub-band precoding, and per-sub-bandprecoding, may also be included.

PMI selection may be based on various criteria. Several criteria of PMIselection include minimum mean square error (MMSE) based, channelcapacity based and correlation based criteria.

For each PMI of the plurality of PMIs, the value of a metric indicativeof a channel condition of the MIMO communication by the WTRU using thatPMI is determined, step 600. This metric may be based on one or moremeasured quantities indicative of the channel condition, such assignal-to-interference-and-noise ratio (SINR), throughput, a block errorrate (BLER), system capacity, or a sum rate.

The PMI may then be selected from the plurality of PMIs based on thevalues of the metric, step 602.

For example, if the metric is a sum rate based on SINR, the SINR may becomputed based on the channel measurement, (e.g., channel estimation)for each PMI. Alternatively, the SINR for each PMI on each sub-band(i.e., resource block (RB)), or resource block group (RGB) may bemeasured. A rate for each sub-band, or RBG, may then be computed basedon the measured SINR for the sub-band, or RBG. The overall sum rate isthen computed for all sub-bands for each PMI by adding the sub-bandrates, or RBG rates, of the individual sub-bands, or RBGs. The desiredPMI is selected based on this overall sum rate, i.e. the PMI having thelargest sum rate is selected.

In the following description of an example of the channel conditiondetermination and selection step according to FIG. 6, Ω is used todenote a codebook or a set of candidate precoding matrix F, such as aDiscrete Fourier Transform (DFT) or a Householder (HH) matrix. H andH_(eff) are used to denote channel state information (CSI) and effectiveCSI, respectively. The effective CSI is generated from H for aparticular precoding matrix F by the following equation:

H_(eff)=HF.  Equation (28)

When a MMSE linear detection receiver is used, depending on MMSEdetection form is used, the mean square error (MSE) may be expressed inaccordance with the following equation:

$\begin{matrix}{{{{{MSE}(F)} = \left( {{H_{eff}^{H}H_{eff}} + {\frac{1}{\rho}I}} \right)^{- 1}};}{or}} & {{Equation}\mspace{14mu} \left( {29\; a} \right)} \\{{{{MSE}(F)} = \left( {{H_{eff}H_{eff}^{H}} + {\frac{1}{\rho}I}} \right)^{- 1}},} & {{Equation}\mspace{14mu} \left( {29\; b} \right)}\end{matrix}$

where ρ is the signal-to-noise ratio.

In one approach, the PMI is selected if the corresponding MSE isminimized, i.e., select F if the trace of MSE in Equation (2) isminimized in accordance with:

$\begin{matrix}{F = {\arg \; {\min\limits_{F_{i} \in \Omega}{{{tr}\left( {{MSE}\left( F_{i} \right)} \right)}.}}}} & {{Equation}\mspace{14mu} (30)}\end{matrix}$

Channel capacity for a given H and F (or the effective channel H_(eff))may be expressed for two different linear minimum mean square error(LMMSE) forms by:

$\begin{matrix}{{{{{Capacity}(F)} = {\log_{2}{\det\left( {{H_{eff}^{H}H_{eff}} + {\frac{1}{\rho}I}} \right)}}};}{or}} & {{Equation}\mspace{14mu} \left( {31\; a} \right)} \\{{{Capacity}(F)} = {\log_{2}{{\det\left( {{H_{eff}H_{eff}^{H}} + {\frac{1}{\rho}I}} \right)}.}}} & {{Equation}\mspace{14mu} \left( {31\; b} \right)}\end{matrix}$

In another approach, the PMI is selected to maximize the channelcapacity, i.e., select F by the following rule:

$\begin{matrix}{F = {\arg \; {\min\limits_{F_{i} \in \Omega}{{{Capacity}\left( F_{i} \right)}.}}}} & {{Equation}\mspace{14mu} (32)}\end{matrix}$

A further approach for selecting the PMI involves estimating the channelresponses H and performing a singular value decomposition (SVD) on theestimated H to obtain a precoding matrix V. For N streams MIMOtransmission where 1≦N≦N_(t), let A be the sub-matrix of V that is usedto precode the N stream data. Furthermore let B_(i) be the possiblecombinations of N column vectors of matrix F. Search all the possiblecombinations of column vectors of F, i.e., all the possible B_(i) andfind the one which maximize the sum of norm of the inner product orcorrelation of A and B_(i) in the search such that:

$\begin{matrix}{T = {\max\limits_{B_{i}}{\sum\limits_{j = 1}^{N}{{{{< {A\left( {:{,j}} \right)}^{*}},{{B_{i}\left( {:{,j}} \right)} >}}}.}}}} & {{Equation}\mspace{14mu} (33)}\end{matrix}$

Precoding matrix selection may also be performed based on sum ratecriteria. SINR for LMMSE may be computed for each precoding matrix F.Depending on the type of LMMSE used, the SINR for the j-th spatialstream for precoding matrix F_(i) may be expressed by:

$\begin{matrix}{{{{\gamma_{g}^{(j)}\left( F_{i} \right)} = {\frac{1}{{\sigma_{n}^{2}\left\lbrack \left( {{H\; F_{i}F_{i}^{H}H^{H}} + {\sigma_{n}^{2}I}} \right\rbrack^{- 1} \right\rbrack}_{j,j}} - 1}};}{or}} & {{Equation}\mspace{14mu} \left( {34\; a} \right)} \\{{{\gamma_{g}^{(j)}\left( F_{i} \right)} = {\frac{1}{{\sigma_{n}^{2}\left\lbrack \left( {{F_{i}^{H}H^{H}{HF}_{i}} + {\sigma_{n}^{2}I}} \right\rbrack^{- 1} \right\rbrack}_{j,j}} - 1}},} & {{Equation}\mspace{14mu} \left( {34\; b} \right)}\end{matrix}$

where H is the average channel matrix within a resource block group(RBG).

The sum rate for the i-th precoding matrix F_(i) and for the g-th RBGmay be expressed by:

$\begin{matrix}{\; {{{\prod\limits_{g}^{\;}\; \left( F_{i} \right)} = {\sum\limits_{j = 1}^{Ns}{\log \; 2\left( {1 + {\gamma_{g}^{(j)}\left( F_{i} \right)}} \right)}}},}} & {{Equation}\mspace{14mu} (35)}\end{matrix}$

where Ns is the number of data streams. The precoding matrix Fi thatmaximizes sum rate is selected,

$\begin{matrix}{F_{sel} = {\arg \; {\max\limits_{F_{i} \in \Omega}{\prod\limits_{g}^{\;}\; {\left( F_{i} \right).}}}}} & {{Equation}\mspace{14mu} (36)}\end{matrix}$

SINR for LMMSE may be computed for each precoding matrix F_(i) and foreach spatial stream. The SINR for the j-th spatial stream for precodingmatrix F_(i) may be expressed by:

$\begin{matrix}{{\gamma_{g}^{(j)} = \frac{{w_{g,j,j}}^{2}}{{\sum\limits_{{k = 1},{k \neq j}}^{Ns}{w_{g,j,k}}^{2}} + {\sigma_{n}^{2}{\sum\limits_{k = 1}^{Ns}{z_{g,j,k}}^{2}}}}},} & {{Equation}\mspace{14mu} (37)}\end{matrix}$

where w_(g,j,k) and z_(g,j,k) is the (j,k)-th element of matrix W_(g)and Z_(g) respectively.

Matrix Z_(g) may be obtained by:

Z _(g) =H _(eff,g) ^(H)(H _(eff,g) H _(eff,g) ^(H) +R _(In))⁻¹  Equation(38)

where R_(In) is the covariance matrix of interference and noise.

The matrix W_(g) may be obtained by:

W_(g)=Z_(g)H_(eff,g).  Equation (39)

The effective channel matrix H_(eff,g) for the i-th precoding matrix Fiand the g-th RBG may be obtained by:

H_(eff,g)=H_(g)F_(i).  Equation (40)

The sum rate for the i-th precoding matrix F_(i) and for the g-th RBGmay be expressed by:

$\begin{matrix}{{{\prod\limits_{g}^{\;}\left( F_{i} \right)} = {\sum\limits_{j = 1}^{Ns}{\log \; 2\left( {1 + {\gamma_{g}^{(j)}\left( F_{i} \right)}} \right)}}},} & {{Equation}\mspace{14mu} (41)}\end{matrix}$

where Ns is the number of data streams. The precoding F_(i) thatmaximizes sum rate for the g-th RBG is selected,

$\begin{matrix}{F_{{sel},g} = {\arg \mspace{11mu} {\max\limits_{F_{i} \in \Omega}{\prod\limits_{g}\; {\left( F_{i} \right).}}}}} & {{Equation}\mspace{14mu} (42)}\end{matrix}$

SINR for MMSE-SIC may be computed for each precoding matrix F_(i) andfor each spatial stream j. The SINR for the codeword that is detectedfirst is the same as SINR for MMSE. The SINR for the j-th spatial streamfor precoding matrix F_(i) is shown in Equation (37).

The post detection SINR depends on the detection order of SIC. For MIMO2×2 if detection starts with codeword 1 (CW1 or the first layer), theSINR for CW1 is computed using Equation (37) letting j=1.

For CW2 (or the second layer) the interference cancellation is takeninto account. Assume SIC perfectly removes the interference arising fromCW1. The equation is reduced to rank one transmission instead of ranktwo. The effective channel matrix H_(eff,g) for the i-th precodingmatrix F_(i) and the g-th RBG becomes a 2×1 matrix and may be obtainedby:

H _(eff,g) ² =H _(g) F _(i)(:,2),  Equation (43)

where F_(i)(:,2) indicates the second column vector of matrix F_(i).

Matrix Z_(g) ⁽²⁾ with dimension of 1×2 may be obtained by:

Z _(g) ⁽²⁾ =H _(eff,g) ⁽²⁾ ^(H) (H _(eff,g) ⁽²⁾ H _(eff,g) ⁽²⁾ ^(H) +R_(In))⁻¹.  Equation (44)

Scalar W_(g) ⁽²⁾ may be obtained by:

W_(g) ⁽²⁾=Z_(eff,g) ⁽²⁾H_(eff,g) ⁽²⁾.  Equation (45)

The SINR for the 2nd spatial stream for precoding matrix F_(i) may beexpressed by:

$\begin{matrix}{{\gamma_{g}^{(2)} = \frac{{w_{g,j,j}^{(2)}}^{2}}{\sigma_{n}^{2}{\sum\limits_{k = 1}^{Ns}{z_{g,j,k}^{(2)}}^{2}}}},} & {{Equation}\mspace{14mu} (46)}\end{matrix}$

where w_(g,j,k) ⁽²⁾ and z_(g,j,k) ⁽²⁾ is the (j,k)-th element of matrixW_(g) ⁽²⁾ and Z_(g) ⁽²⁾ respectively.

The sum rate for the i-th precoding matrix F_(i) and for the g-th RBGmay be expressed by:

$\begin{matrix}{{{\prod\limits_{g}\; \left( F_{i} \right)} = {\sum\limits_{j = 1}^{Ns}{\log \; 2\left( {1 + {\gamma_{g}^{(j)}\left( F_{i} \right)}} \right)}}},} & {{Equation}\mspace{14mu} (47)}\end{matrix}$

where Ns is the number of data streams. The precoding matrix F_(i) thatmaximizes sum rates for the g-th RBG is selected,

$\begin{matrix}{F_{{sel},g} = {\arg \mspace{11mu} {\max\limits_{F_{i} \in \Omega}{\prod\limits_{g}\; {\left( F_{i} \right).}}}}} & {{Equation}\mspace{14mu} (48)}\end{matrix}$

From Equations (37), (46) and (47) it may be observed that the SICdetection order affects the calculated SINR for the first and second CW.If the 1st CW is processed first and then the 2nd CW is detected, wehave γ_(g) ⁽¹⁾ and γ_(g) ⁽²⁾. If the 2nd CW is processed first and thenthe 1^(st) CW is detected, we have γ_(g) ⁽¹⁾′ and γ_(g) ⁽²⁾′. This isbecause the first processed CW only uses MMSE detection. SINR depends onboth CWs signal strength and the interference arising from the otherCWs. Choosing different CW as the CW being processed first results indifferent SINR. Furthermore, SINR for the CW takes into account of SICand thus depends only on its own signal strength and noise level, notinterference from the other CW. This is based on the post detectionSINR.

The sum rates are computed for each F_(i) for [γ_(g) ⁽¹⁾, γ_(g) ⁽²⁾] and[γ_(g) ⁽¹⁾′, γ_(g) ⁽²⁾′] as follows:

$\begin{matrix}{{{{\prod\limits_{g}\; \left( F_{i} \right)} = {\sum\limits_{j = 1}^{2}{\log \; 2\left( {1 + {\gamma_{g}^{(j)}\left( F_{i} \right)}} \right)}}};}{and}} & {{Equation}\mspace{14mu} (49)} \\{{\overset{\prime}{\prod\limits_{g}\;}\left( F_{i} \right)} = {\sum\limits_{j = 1}^{2}{\log \; 2{\left( {1 + {\gamma_{g}^{{(j)}^{\prime}}\left( F_{i} \right)}} \right).}}}} & {{Equation}\mspace{14mu} (50)}\end{matrix}$

The computed sum rates, Π_(g)(F_(i)) and Π_(g)′(F_(i)) in Equations (49)and (50), are different from each other for different detection order ofSIC. The SIC detection order may be determined based on criteria thatthe sum rates in Equations (49) and (50) are maximized. The detectionorder may be selected if the overall rates for the given detection orderis greatest. Rank, PMI, CQI, or combinations of rank, PMI and/or CQI maybe generated accordingly using the approaches described previously.

In wideband precoding, a single precoding matrix is generated for theentire bandwidth. The size of RBG is chosen such that the averagedchannel responses are sufficiently good. Once the RBG size isdetermined, the entire bandwidth may be divided into multiple RBGs, sayN_(G). In each RBG a rate may be calculated accordingly. The sum ratefor the i-th precoding matrix F_(i) and for the g-th RBG may beexpressed by:

$\begin{matrix}{{{\prod\limits_{g}\; \left( F_{i} \right)} = {\sum\limits_{j = 1}^{Ns}{\log \; 2\left( {1 + {\gamma_{g}^{(j)}\left( F_{i} \right)}} \right)}}},} & {{Equation}\mspace{14mu} (51)}\end{matrix}$

where Ns is the number of data streams. The overall rate of the entirebandwidth is sum of all of individual rate of each RBG as:

$\begin{matrix}{{{\prod\limits_{T}\; \left( F_{i} \right)} = {\sum\limits_{g = 1}^{N_{G}}{\prod\limits_{g}\; \left( F_{i} \right)}}},} & {{Equation}\mspace{14mu} (52)}\end{matrix}$

where N_(G) is the number of RBGs for the entire bandwidth. Theprecoding matrix F_(i) that maximizes sum rate for the entire bandwidthis selected,

$\begin{matrix}{F_{sel} = {\arg \mspace{11mu} {\max\limits_{F_{i} \in \Omega}{\prod\limits_{T}\; {\left( F_{i} \right).}}}}} & {{Equation}\mspace{14mu} (53)}\end{matrix}$

In M-sub-band precoding a single precoding matrix is generated for the Mpreferred sub-bands in the given bandwidth. A given bandwidth is a setof sub-bands S, i.e., the M preferred sub-bands within the set ofsub-bands S. M-sub-band precoding corresponds to WTRU selected sub-bandsprecoding and feedback. The size of RBG is chosen such that the averagedchannel responses are sufficiently good. Once the RBG size isdetermined, each sub-band may be divided into one or multiple RBGs, sayG_(S). In each RBG a rate may be calculated accordingly. Assume N_(S)sub-bands in a set of sub-bands S. There are totally Q combinations forM-sub-bands when choosing M sub-bands among N_(S) sub-bands, such as:

Q=C_(M) ^(N) ^(S) .  Equation (54)

The sum rate for the i-th precoding matrix F_(i) and for the g-th RBG inthe q-th combination may be expressed by:

$\begin{matrix}{{{\overset{q}{\prod\limits_{g}}\; \left( F_{i} \right)} = {\sum\limits_{j = 1}^{Ns}{\log \; 2\left( {1 + {\gamma_{g}^{(j)}\left( F_{i} \right)}} \right)}}},} & {{Equation}\mspace{14mu} (55)}\end{matrix}$

where Ns is the number of data streams. The rate of each of the Qcombination (I.e., each M-sub-band) is sum of all of individual rate ofeach RBG in each sub-band and sum of all the M sub-bands. For the q-thcombination the rate is calculated as

$\begin{matrix}{{\overset{q}{\prod\limits_{T}}\; \left( F_{i} \right)} = {\sum\limits_{g = 1}^{M \cdot G_{s}}{\overset{q}{\prod\limits_{g}}\; {\left( F_{i} \right).}}}} & {{Equation}\mspace{14mu} (56)}\end{matrix}$

Denoting the set of Q combination as Ω_(Q), the precoding matrix F_(i)and the q-th combination that maximizes overall rate for the M sub-bandsare jointly selected in accordance with the following equation:

$\begin{matrix}{\left( {F_{sel},b} \right) = {\arg \mspace{11mu} {\max\limits_{\underset{q \in \Omega_{Q}}{{F_{i} \in \Omega},}}{\prod\limits_{T}^{q}\; {\left( F_{i} \right).}}}}} & {{Equation}\mspace{14mu} (57)}\end{matrix}$

In per-sub-band precoding a single precoding matrix is generated foreach sub-band in a set of sub-bands S. Per-sub-band precodingcorresponds to multiple precoding feedback. The size of RBG is chosensuch that the averaged channel responses are sufficiently good. Once theRBG size is determined, each sub-band may be divided into one ormultiple RBGs, say G_(S). In each RBG a rate may be calculatedaccordingly. Assume N_(S) sub-bands in a set of sub-bands S. The sumrate for the i-th precoding matrix F_(i) and for the g-th RBG may beexpressed as:

$\begin{matrix}{{{\prod\limits_{g}\; \left( F_{i} \right)} = {\sum\limits_{j = 1}^{Ns}{\log \; 2\left( {1 + {\gamma_{g}^{(j)}\left( F_{i} \right)}} \right)}}},} & {{Equation}\mspace{14mu} (58)}\end{matrix}$

where Ns is the number of data streams. The rate of each sub-band is sumof all of individual rate of RBG in each sub-band as:

$\begin{matrix}{{\prod\limits_{S}\; \left( F_{i} \right)} = {\sum\limits_{g = 1}^{G_{s}}{\prod\limits_{g}\; {\left( F_{i} \right).}}}} & {{Equation}\mspace{14mu} (59)}\end{matrix}$

The precoding matrix F_(i) that maximizes rate of the sub-band isselected in accordance with:

$\begin{matrix}{F_{sel} = {\arg \mspace{11mu} {\max\limits_{F_{i} \in \Omega}{\prod\limits_{S}\; {\left( F_{i} \right).}}}}} & {{Equation}\mspace{14mu} (60)}\end{matrix}$

An example codebook of containing eight DFT matrix with different phaseshifts is as follows:

${F_{{4 \times 4},0} = \begin{bmatrix}1 & 1 & 1 & 1 \\1 & ^{j\frac{1}{2}\pi} & ^{j\pi} & ^{{- j}\frac{1}{2}\pi} \\1 & ^{j\pi} & ^{j2\pi} & ^{j\pi} \\1 & ^{{- j}\frac{1}{2}\pi} & ^{j\pi} & ^{j\frac{1}{2}\pi}\end{bmatrix}};$ ${F_{{4 \times 4},1} = \begin{bmatrix}1 & 1 & 1 & 1 \\^{j\frac{1}{16}\pi} & ^{j\frac{9}{16}\pi} & ^{{- j}\frac{15}{16}\pi} & ^{{- j}\frac{7}{16}\pi} \\^{j\frac{1}{8}\pi} & ^{{- j}\frac{7}{8}\pi} & ^{j\frac{1}{8}\pi} & ^{{- j}\frac{7}{8}\pi} \\^{j\frac{3}{16}\pi} & ^{{- j}\frac{5}{16}\pi} & ^{{- j}\frac{13}{16}\pi} & ^{j\frac{11}{16}\pi}\end{bmatrix}};$ ${F_{{4 \times 4},2} = \begin{bmatrix}1 & 1 & 1 & 1 \\^{j\frac{1}{8}\pi} & ^{j\frac{5}{8}\pi} & ^{{- j}\frac{7}{8}\pi} & ^{{- j}\frac{3}{8}\pi} \\^{j\frac{1}{4}\pi} & ^{{- j}\frac{3}{4}\pi} & ^{j\frac{1}{4}\pi} & ^{{- j}\frac{3}{4}\pi} \\^{j\frac{3}{8}\pi} & ^{{- j}\frac{1}{8}\pi} & ^{{- j}\frac{5}{8}\pi} & ^{j\frac{7}{8}\pi}\end{bmatrix}};$ ${F_{{4 \times 4},3} = \begin{bmatrix}1 & 1 & 1 & 1 \\^{j\frac{3}{16}\pi} & ^{j\frac{11}{16}\pi} & ^{{- j}\frac{13}{16}\pi} & ^{{- j}\frac{5}{16}\pi} \\^{j\frac{3}{8}\pi} & ^{{- j}\frac{5}{8}\pi} & ^{j\frac{3}{8}\pi} & ^{{- j}\frac{5}{8}\pi} \\^{j\frac{9}{16}\pi} & ^{j\frac{1}{16}\pi} & ^{{- j}\frac{7}{16}\pi} & ^{{- j}\frac{15}{16}\pi}\end{bmatrix}};$ ${F_{{4 \times 4},4} = \begin{bmatrix}1 & 1 & 1 & 1 \\^{j\frac{1}{4}\pi} & ^{j\frac{3}{4}\pi} & ^{{- j}\frac{3}{4}\pi} & ^{{- j}\frac{1}{4}\pi} \\^{j\frac{1}{2}\pi} & ^{{- j}\frac{1}{2}\pi} & ^{j\frac{1}{2}\pi} & ^{{- j}\frac{1}{2}\pi} \\^{j\frac{3}{4}\pi} & ^{j\frac{1}{4}\pi} & ^{{- j}\frac{1}{4}\pi} & ^{j\frac{5}{4}\pi}\end{bmatrix}};$ ${F_{{4 \times 4},5} = \begin{bmatrix}1 & 1 & 1 & 1 \\^{j\frac{5}{16}\pi} & ^{j\frac{13}{16}\pi} & ^{{- j}\frac{11}{16}\pi} & ^{{- j}\frac{3}{16}\pi} \\^{j\frac{5}{8}\pi} & ^{{- j}\frac{3}{8}\pi} & ^{j\frac{5}{8}\pi} & ^{{- j}\frac{3}{8}\pi} \\^{j\frac{15}{16}\pi} & ^{j\frac{7}{16}\pi} & ^{{- j}\frac{1}{16}\pi} & ^{{- j}\frac{9}{16}\pi}\end{bmatrix}};$ ${F_{{4 \times 4},6} = \begin{bmatrix}1 & 1 & 1 & 1 \\^{j\frac{3}{8}\pi} & ^{j\frac{7}{8}\pi} & ^{{- j}\frac{5}{8}\pi} & ^{{- j}\frac{1}{8}\pi} \\^{j\frac{3}{4}\pi} & ^{{- j}\frac{1}{4}\pi} & ^{j\frac{3}{4}\pi} & ^{{- j}\frac{1}{4}\pi} \\^{j\frac{9}{8}\pi} & ^{j\frac{5}{8}\pi} & ^{j\frac{1}{8}\pi} & ^{{- j}\frac{3}{8}\pi}\end{bmatrix}};$ $F_{{4 \times 4},7} = {\begin{bmatrix}1 & 1 & 1 & 1 \\^{j\frac{7}{16}\pi} & ^{j\frac{15}{16}\pi} & ^{{- j}\frac{9}{16}\pi} & ^{{- j}\frac{1}{16}\pi} \\^{j\frac{7}{8}\pi} & ^{{- j}\frac{1}{8}\pi} & ^{j\frac{7}{8}\pi} & ^{{- j}\frac{1}{8}\pi} \\^{{- j}\frac{11}{16}\pi} & ^{j\frac{13}{16}\pi} & ^{j\frac{5}{16}\pi} & ^{{- j}\frac{3}{16}\pi}\end{bmatrix}.}$

Table 1 below shows an example codebook:

TABLE 3 Codebook Number of layers υ index u_(n) 1 2 3 4 0 u₀ = [1 −1 −1−1]^(T) w₀ ^({1}) W₀ ^({14})/{square root over (2)} W₀ ^({124})/{squareroot over (3)} W₀ ^({1234})/2 1 u₁ = [1 −j 1 j]^(T) W₁ ^({1}) W₁^({12})/{square root over (2)} W₁ ^({123})/{square root over (3)} W₁^({1234})/2 2 u₂ = [1 1 −1 1]^(T) W₂ ^({1}) W₂ ^({12})/{square root over(2)} W₂ ^({123})/{square root over (3)} W₂ ^({3214})/2 3 u₃ = [1 j 1−j]^(T) W₃ ^({1}) W₃ ^({12})/{square root over (2)} W₃ ^({123})/{squareroot over (3)} W₃ ^({3214})/2 4 u₄ = [1 (−1 − j)/{square root over (2)}−j (1 − j)/{square root over (2)}]^(T) W₄ ^({1}) W₄ ^({14})/{square rootover (2)} W₄ ^({124})/{square root over (3)} W₄ ^({1234})/2 5 u₅ = [1 (1− j)/{square root over (2)} j (−1 − j)/{square root over (2)}]^(T) W₅^({1}) W₅ ^({14})/{square root over (2)} W₅ ^({124})/{square root over(3)} W₅ ^({1234})/2 6 u₆ = [1 (1 + j)/{square root over (2)} −j (−1 +j)/{square root over (2)}]^(T) W₆ ^({1}) W₆ ^({13})/{square root over(2)} W₆ ^({134})/{square root over (3)} W₆ ^({1324})/2 7 u₇ = [1 (−1 +j)/{square root over (2)} j (1 + j)/{square root over (2)}]^(T) W₇^({1}) W₇ ^({13})/{square root over (2)} W₇ ^({134})/{square root over(3)} W₇ ^({1324})/2 8 u₈ = [1 −1 1 1]^(T) W₈ ^({1}) W₈ ^({12})/{squareroot over (2)} W₈ ^({124})/{square root over (3)} W₈ ^({1234})/2 9 u₉ =[1 −j −1 −j]^(T) W₉ ^({1}) W₉ ^({14})/{square root over (2)} W₉^({134})/{square root over (3)} W₉ ^({1234})/2 10 u₁₀ = [1 1 1 −1]^(T)W₁₀ ^({1}) W₁₀ ^({13})/{square root over (2)} W₁₀ ^({123})/{square rootover (3)} W₁₀ ^({1324})/2 11 u₁₁ = [1 j −1 j]^(T) W₁₁ ^({1}) W₁₁^({13})/{square root over (2)} W₁₁ ^({134})/{square root over (3)} W₁₁^({1324})/2 12 u₁₂ = [1 −1 −1 1]^(T) W₁₂ ^({1}) W₁₂ ^({12})/{square rootover (2)} W₁₂ ^({123})/{square root over (3)} W₁₂ ^({1234})/2 13 u₁₃ =[1 −1 1 −1]^(T) W₁₃ ^({1}) W₁₃ ^({13})/{square root over (2)} W₁₃^({123})/{square root over (3)} W₁₃ ^({1324})/2 14 u₁₄ = [1 1 −1 −1]^(T)W₁₄ ^({1}) W₁₄ ^({13})/{square root over (2)} W₁₄ ^({123})/{square rootover (3)} W₁₄ ^({3214})/2 15 u₁₅ = [1 1 1 1]^(T) W₁₅ ^({1}) W₁₅^({12})/{square root over (2)} W₁₅ ^({123})/{square root over (3)} W₁₅^({1234})/2

Once the desired PMI for MIMO communication by the WTRU has beenselected, the selected PMI is transmitted by the WTRU, step 604. IfM-sub-band precoding is used, then transmitting the selected PMIincludes transmitting the selected combination of M sub-bands and theselected PMI. And if separate PMIs are selected for each sub-band, thentransmitting the selected PMI includes transmitting the selectedsub-band PMI for each sub-band.

Although features and elements are described above in particularcombinations, each feature or element may be used alone without theother features and elements or in various combinations with or withoutother features and elements. The methods or flow charts provided hereinmay be implemented in a computer program, software, or firmwareincorporated in a computer-readable storage medium for execution by ageneral purpose computer or a processor. Examples of computer-readablestorage mediums include a read only memory (ROM), a random access memory(RAM), a register, cache memory, semiconductor memory devices, magneticmedia such as internal hard disks and removable disks, magneto-opticalmedia, and optical media such as CD-ROM disks, and digital versatiledisks (DVDs).

Suitable processors include, by way of example, a general purposeprocessor, a special purpose processor, a conventional processor, adigital signal processor (DSP), a plurality of microprocessors, one ormore microprocessors in association with a DSP core, a controller, amicrocontroller, Application Specific Integrated Circuits (ASICs), FieldProgrammable Gate Arrays (FPGAs) circuits, any other type of integratedcircuit (IC), and/or a state machine.

A processor in association with software may be used to implement aradio frequency transceiver for use in a wireless transmit receive unit(WTRU), user equipment (UE), terminal, base station, radio networkcontroller (RNC), or any host computer. The WTRU may be used inconjunction with modules, implemented in hardware and/or software, suchas a camera, a video camera module, a videophone, a speakerphone, avibration device, a speaker, a microphone, a television transceiver, ahands free headset, a keyboard, a Bluetooth® module, a frequencymodulated (FM) radio unit, a liquid crystal display (LCD) display unit,an organic light-emitting diode (OLED) display unit, a digital musicplayer, a media player, a video game player module, an Internet browser,and/or any wireless local area network (WLAN) or Ultra Wide Band (UWB)module.

1. A method for selecting at least one rank from a plurality of ranksfor multiple-input multiple-output (MIMO) communication by a wirelesstransmitter/receiver unit (WTRU), the method comprising: for each rankof the plurality of ranks, determining a value of a metric indicative ofa channel condition of the MIMO communication by the WTRU using thatrank; selecting the at least one rank from the plurality of ranks basedon the values of the metric; and transmitting rank informationindicative of the at least one selected rank for the MIMO communicationby the WTRU.
 2. The method of claim 1, wherein determining the value ofthe metric for each rank of the plurality of ranks includes: for eachspatial stream used for the MIMO communication, determining a streamsub-value of the metric; and adding the stream sub-values of the metricto determine the value of the metric.
 3. The method of claim 1, whereindetermining the value of the metric for each rank of the plurality ofranks includes: for each radio block group (RBG) used for the MIMOcommunication, determining an RBG sub-value of the metric; and addingthe RBG sub-values of the metric to determine the value of the metric.4. The method of claim 1, wherein: the metric is a sum rate; the atleast one rank is one rank; the MIMO communication uses successiveinterference cancellation (SIC) based on a set of codewords equal innumber to a number of layers used by the WTRU for each rank; determiningthe sum rate for each rank of the plurality of ranks includes:determining an SIC order sum rate for the rank for each ordering ofcodewords in the set of codewords by: measuring asignal-to-interference-and-noise ratio (SINR) for each layer used by theWTRU using SIC; calculating a layer rate for each layer used by the WTRUfrom the measured SINR for the layer; and adding the layer rates todetermine the SIC order rate; selecting the highest SIC order rate forthe rank to determine the sum rate for the rank and the selectedordering of codewords in the set of codewords for that rank; selectingthe at least one rank from the plurality of ranks includes selecting theone rank having the highest sum rate; and transmitting the selectedordering of codewords in the set of codewords of the one selected rank.5. The method of claim 1, wherein the rank information is transmitted ona physical uplink shared channel (PUSCH).
 6. The method of claim 5,wherein the rank information is transmitted aperiodically.
 7. The methodof claim 1, further comprising: for each precoding matrix index (PMI) ofa plurality of PMIs, determining a PMI value of the metric using each ofthe at least one selected rank; selecting one PMI from the plurality ofPMIs based on the PMI values of the metric for each of the at least oneselected rank; and transmitting the selected PMI for each of the atleast one selected rank.
 8. The method of claim 1, wherein: the metricis a sum rate; the at least one rank is two ranks; and selecting the atleast one rank from the plurality of ranks includes selecting the tworanks having the two highest sum rates.
 9. A method for joint selectionof at least one precoding matrix index (PMI) from a plurality of PMIsand at least one rank from a plurality of ranks for multiple-inputmultiple-output (MIMO) communication by a wireless transmitter/receiverunit (WTRU), the method comprising: for each PMI and rank combination,determining a value of a metric indicative of a channel condition of theMIMO communication by the WTRU using that PMI and rank; selecting atleast one rank and PMI combination based on the values of the metric;and transmitting the PMI and rank information indicative of the rank foreach of the at least one selected rank and PMI combination for the MIMOcommunication by the WTRU.
 10. The method of claim 9, whereindetermining the value of the metric for each PMI and rank combinationincludes: for each radio block group (RBG) used for the MIMOcommunication, determining an RBG sub-value of the metric; and addingthe RBG sub-values of the metric to determine the value of the metric.11. The method of claim 9, wherein: the metric is a sum rate; the atleast one PMI and rank combination is one PMI and rank combination; theMIMO communication uses successive interference cancellation (SIC) basedon a set of codewords equal in number to a number of layers used by theWTRU for each rank; determining the sum rate for each PMI and rankcombination includes: determining an SIC order sum rate for the PMI andrank combination for each ordering of codewords in the set of codewordsby: measuring a signal-to-interference-and-noise ratio (SINR) for eachlayer used by the WTRU using SIC; calculating a layer rate for eachlayer used by the WTRU from the measured SINR for the layer; and addingthe layer rates to determine the SIC order rate; selecting the highestSIC order rate for the PMI and rank combination to determine the sumrate for the PMI and rank combination and the selected ordering ofcodewords in the set of codewords for that PMI and rank combination; andselecting the at least one PMI and rank combination includes selectingthe one PMI and rank combination having the highest sum rate; andtransmitting the selected ordering of codewords in the set of codewordsof the one selected PMI and rank combination.
 12. The method of claim 9,wherein the PMI and the rank information are transmitted on a physicaluplink shared channel (PUSCH).
 13. The method of claim 12, wherein thePMI and the rank information are transmitted aperiodically.
 14. Themethod of claim 9, wherein: the metric is a sum rate; the at least onePMI and rank combination is two PMI and rank combinations; and selectingthe at least one PMI and rank combination includes selecting the two PMIand rank combinations having the two highest sum rates.
 15. A method forselecting at least one precoding matrix index (PMI) from a plurality ofPMIs for multiple-input multiple-output (MIMO) communication by awireless transmitter/receiver unit (WTRU), the method comprising: foreach PMI of the plurality of PMIs, determining a value of a metricindicative of a channel condition of the MIMO communication by the WTRUusing that PMI; selecting the at least one PMI from the plurality ofPMIs based on the values of the metric; and transmitting the at leastone selected PMI for the MIMO communication by the WTRU.
 16. The methodof claim 15, wherein: the metric is a sum rate; the at least one PMI isone PMI; the MIMO communication uses successive interferencecancellation (SIC) based on a set of codewords equal in number to anumber of layers used by the WTRU; determining the sum rate for each PMIincludes: determining an SIC order sum rate for the PMI for eachordering of codewords in the set of codewords by: measuring asignal-to-interference-and-noise ratio (SINR) for each layer used by theWTRU using SIC; calculating a layer rate for each layer used by the WTRUfrom the measured SINR for the layer; and adding the layer rates todetermine the SIC order rate; selecting the highest SIC order rate forthe PMI to determine the sum rate for the PMI and the selected orderingof codewords in the set of codewords for that PMI; and selecting the atleast one PMI from the plurality of PMIs includes selecting the one PMIhaving the highest sum rate; and transmitting the selected ordering ofcodewords in the set of codewords of the one selected PMI.
 17. Themethod of claim 15, wherein: the metric is a sum rate; the at least onePMI is two PMIs; and selecting the at least one PMI from the pluralityof PMIs includes selecting the two PMIs having the two highest sumrates.
 18. A wireless transmitter/receiver unit (WTRU) configured formultiple-input multiple-output (MIMO) communication, the WTRUcomprising: a receiver to receive a signal; a channel conditionprocessor coupled to the receiver, for each rank of a plurality ofranks, the channel condition processor configured to determine a valueof a metric indicative of a channel condition of that rank based on thereceived signal; a selection processor coupled to the channel conditionprocessor, the selection processor configured to select at least onerank from the plurality of ranks based on the values of the metric; atransmitter coupled to the selection processor, the transmitterconfigured to transmit rank information indicative of the at least oneselected rank.
 19. A wireless transmitter/receiver unit (WTRU)configured for multiple-input multiple-output (MIMO) communication,including joint selection of at least one precoding matrix index (PMI)and rank combination from a plurality of PMIs and rank combinations, theWTRU comprising: a receiver to receive a signal; a channel conditionprocessor coupled to the receiver, for each PMI and rank combination,the channel condition processor configured to determine a value of ametric indicative of a channel condition of that PMI and rankcombination based on the received signal; a joint selection processorcoupled to the channel condition processor, the selection processorconfigured to select at least one rank and PMI combination based on thevalues of the metric; a transmitter coupled to the joint selectionprocessor, the transmitter configured to transmit the PMI and rankinformation indicative of the rank for each of the at least one selectedrank and PMI combination.
 20. A wireless transmitter/receiver unit(WTRU) configured for multiple-input multiple-output (MIMO)communication, the WTRU comprising: a receiver to receive a signal; achannel condition processor coupled to the receiver, for each precodingmatrix index (PMI) of a plurality of PMIs, the channel conditionprocessor configured to determine a value of a metric indicative of achannel condition using that PMI based on the received signal; aselection processor coupled to the channel condition processor, theselection processor configured to select at least one PMI from theplurality of PMIs based on the values of the metric; a transmittercoupled to the selection processor, the transmitter configured totransmit the at least one selected PMI.